What Causes the Tides?

by Gretchen Noyes-Hull

In most places, the oceans rise and fall every 12 hours and 26 minutes. These two daily “high tides” are caused by the gravity of the Moon, and also to a lesser degree by that of the Sun. One of the two high tides is usually greater than the other. Here's why.

The Moon's gravity pulls the oceans on the side of the Earth nearer to it more than it pulls the Earth itself, causing a bulge of water. On the opposite side (away from the Moon), the Earth is pulled more than the oceans, and this leaves behind a slightly smaller bulge of water. So the reason for the difference in the two high tide heights is that the force of gravity gets weaker the farther away a point on the Earth is from the Moon (the inverse square law of gravity). Since the Earth rotates once on its axis every 24 hours, its surface passes through these two bulges, or high tides, each day.

The Sun also has a tidal effect on Earth, but it is only half as much as the Moon's. Since the Sun is so far away, its force does not change very much from one side of the Earth to the other. So the greatest effect from the Sun is seen during New Moon, when the Sun and Moon are in a direct line as seen from Earth, and pulling together on the Earth's oceans. These high tides are called spring tides.

When the Sun and the Moon are at right angles and pulling in different directions, the lowest tides take place, called neap tides.

Activity

What would the effect on the tides be if Earth had no Moon? Why?[anno: The change in the height of the ocean would be very small between tides because the gravitational effect of the Sun on the tides is much smaller than that of the Moon.]

Imagine that there were no tides. What effect might this have on the oceans? What effect might it have on coastal ecosystems? On people? Write a paragraph describing the effect that an absence of tides would have on the Earth.[anno: Answers will vary but could include that without tides, lots of ecosystems around the coast, such as estuaries, would not have their water recycled. The ground would never appear in certain areas, and animals that feed on plants and animals that are unprotected during low tide would suffer.]